1. Field of the Invention
This invention relates in general to spray nozzles and methods of manufacturing same. More particularly it relates to nozzles which produce fine droplet sprays by means of liquid pressure-swift, commonly known as simplex nozzles, and the methods of manufacturing these nozzles.
2. Description of the Prior Art
The art of producing sprays by pressure-swirl is extensive. Generally these nozzles create a vortex in the liquid to be sprayed within a swirl chamber adjacent to the exit or spray orifice. Patents showing such nozzles include U.S. Pat. Nos. 4,613,079 and 4,134,606. However, it is much easier to design and manufacture relatively large spray nozzles for producing relatively larger droplet sprays than to design and manufacture relatively small nozzles to produce relatively fine droplet sprays. This is especially true in the context of manufacturing the inlet slots, swirl chambers, and exit orifices in small nozzles.
One method of characterizing nozzle size is by the dimensions of exit orifice. Small nozzle tips have exit orifices from about 0.005 to about 0.1 inches in diameter. Larger nozzles have larger exit orifice sizes. Another method is the use of "Flow Number," which relates the rate of liquid flow output to the applied inlet pressure by the equation: ##EQU1## In industry the units used are commonly mass flow rate in pounds/hour (PPH) and the applied pressure in pounds/square inch (psi). Thus a spray nozzle which flows 10 lb./hr. at 100 psi has a Flow Number of 1.0. With a given liquid, such as aviation kerosene fuel, the Flow Number is substantially constant over a wide range of flows.
A spray nozzle having a Flow Number of 1.0 typically requires a swirl chamber diameter of 0.075 inch, and exit orifice of 0.012 inch diameter and 2 inlet slots 0.020 inch square or 4 inlet slots 0.014 square. This represents the lower limit of dimensions which can be produced by conventional machining methods. There is a need for spray nozzles with Flow Numbers less than 1.0 down to 0.1, which require even smaller dimensions.
In manufacturing the openings and surfaces of small nozzles it is often necessary to use precision jeweler's tools and microscopes. To manufacture many of these features has heretofore only been possible using relatively low volume machine tool and hand tool operations in connection with high magnification manipulation and examination techniques. This is therefore a labor intensive process with a high rejection or scrap rate. The accuracy with which the dimensions of a nozzle of Flow Number 1.0 can be made limits the consistency of performance of supposedly identical nozzles. For example, if the exit orifice is nominally 0.010 inch diameter, an inaccuracy of only 0.0005 inch (which is about the best that can be achieved by typical manufacturing techniques) will result in a variation in flow rate of 10% from the nominal. Some applications of spray nozzles (e.g., aircraft gas turbine engines) require flow rates to be held within limits of .+-.2%. There is clearly a need for improved methods of manufacture which will give greater accuracy.
Another factor of considerable importance is the need to obtain concentricity of the exit orifice with the swirl chamber and also to place the inlet slots symmetrically relative to the axis of the swirl chamber. This involves the problem of maintaining invariable positioning of the tools and the workpiece, which introduces another set of tolerances or potential inaccuracies. It should be noted also that in the nozzle configuration shown in FIGS. 1 and 2, representing prior art, it is impossible to machine the inlet sots such that they are truly tangential to the outer edge of the swirl chamber.
It is well known that creating a vortex or swift in the liquid to be sprayed from an exit orifice produces finer droplet sizes than would result from a simple jet. This results from the turbulence and tangential shearing forces placed on the thin film of liquid by its swirling motion as it exits the nozzle exit orifice. Generally, faster swirling results in finer droplets.
Finer droplet sizes are desired in a wide range of spray applications. For example, in sprays used in the combustion of fuels, fine droplet sizes improve the efficiency of combustion and reduce the production of undesirable air pollutants.
Another advantage of improved efficiency in droplet formation is that lower pressurization of the liquid can produce the desired size of droplets. In a combustion engine, this allows a lower pressurization of the fuel to result in a spray which is ignitable. This provides many advantages in, for example, an aviation gas turbine engine which uses spray nozzles for combustion of aviation kerosene and which is required to be as simple and light as possible.
Referring now to FIGS. 1 and 2, a spray nozzle 11 constructed in accordance with the prior art is shown. The nozzle 11 is a relatively small nozzle having an exit or spray orifice diameter of approximately 0.020 inches. The spray orifice 13 and the nozzle 11 are of a type suitable for use in an aircraft gas turbine engine. The liquid sprayed by this nozzle would typically be aviation kerosene.
The spray orifice 13 is formed in the cone shaped end 15 of a nozzle housing 17. The interior 19 of the housing 17 is generally cylindrically shaped and has a conical opening 21 which terminates at the spray orifice 13. Retained within the conical opening 21 by a spring 23 is a swirl piece 25.
The swirl piece 25 has an annular wall 27 at its upper end which defines a cylindrical swirl chamber 29 therein. The annular wall 27 contacts the surface of the conical opening 21 so as to form an exit cone 31 between the swirl chamber cavity 29 and the spray orifice 13. The inlets to the swirl chamber 29 are shown through 4 slots 33, 34, 35, and 36 in the annular wall 27 although more or fewer slots can be used. These slots 33, 34, 35 and 36 are directed so that the liquid flowing into the swirl chamber cavity 29 will move in a swirling motion as shown by the arrows 37, 38, 39, and 40 in FIG. 2. Fluid exits the swirl chamber through the exit cone 31 and, in turn, the spray orifice 13.
The liquid proceeds as shown by flow arrows 28 into an annular area 26 formed by the interior 19, the conical opening 21, and the swirl piece 25 by flowing through, in this example, three flats 20, 22, and 24 cut on the swirl piece 25. The liquid is then free to flow through the inlet slots 33, 34, 35, and 36 and into the swirl chamber 29 in such a manner as to create a vortex in said swirl chamber 29.
In order to manufacture the prior art nozzle shown in FIGS. 1 and 2 it is necessary to use very small size cutting and forming tools. Even with very small tools, it is very difficult to accurately form the nozzle and its pieces. For example, it is very difficult to cut the spray orifice 13 both because of the small size of the orifice and because of the need to precisely center the orifice at the tip of the conical opening 21.
It is also difficult to manufacture the swirl piece 25, especially its annular wall 27 and the slots 33, 34, 35 and 36. The annular wall 27 must precisely meet and seal at the edge which contacts the conical opening 21. This may require mate lapping of both surfaces. The slots 33, 34, 35 and 36 require very delicate tools and often hand working under microscopes in order to form them with correct size and position and also to remove burrs which could disrupt flow.
It is therefore an object of the present invention to provide a spray nozzle which is more efficient in its performance and is easier to manufacture. It is also an object of the present invention to provide a configuration and method of manufacture for such nozzles which are especially suited for pressure-swirl nozzles of low Flow Numbers.